1. Field of the Invention
This invention relates to a sensor system for monitoring fluid level and displacement and, more particularly, to a sensor system for monitoring the level of crude oil or crude oil and water in storage containers.
2. Description of Related Art
An inductive level sensor currently used by the oil industry for single fluid applications uses a single toroid shaped float including an LC resonator comprised of a coil and a capacitor imbedded in a buoyant material. In dual fluid applications, such as crude oil and water, the sensor is equipped with two so-called active floats, each comprised of an LC resonator, a switch, a control circuit and a battery powering the circuit imbedded in a buoyant material. Since the circuit is permanently connected to the battery, the life of such float resonator is limited, depending mostly on the temperature of the float resonator in operation, and the type and capacity of the battery. Failure of the float resonator due to the end of life of the battery results in expensive, periodic replacement of both floats. Herein, “float” and “float resonator” may be used interchangeably.
An attempt was made in the past to design a battery-less float resonator based on a large coil with a tap that would produce enough energy to power the float resonator circuit during measurement (see e.g., U.S. Pat. No. 6,474,158, which is incorporated herein by reference). However, the size and, therefore, large weight of the coil required by such circuit made the design not practical for commercialization. Also, the described measurement sequence required large storage and long charging time making the sensor slow and unreliable especially at higher temperatures where power consumption increased due to increased leakage of electronic components and charging ability decreased due to increased resistance of the coil wire. In addition, the dual frequency design disclosed in U.S. Pat. No. 6,474,158 required precise tuning of the resonator circuit. This requirement translated to matching two capacitors and one coil for each float resonator to tolerances that were difficult and expensive to implement. Also the dual frequency design required that during the measuring sequence, the float resonator circuit be powered from the storage capacitor without any additional recharging during the measuring sequence.
It would be desirable to overcome the above problems and others by providing a fluid sensor having one or more active float resonators, each equipped with a power supply that can be energized by the primary coil of the sensor.